Gnotobiotic and germ-free animals are being used in a series of specific experiments to evaluate the role of bacterian in certain gastrointestinal diseases. Specifically, the role of bacteria on the pathogenesis of hepatic encephalopathy, blind loop syndrome and stress ulcer is being investigated utilizing models of these lesions which have been adapted to germ-free and gnotobiotic dogs and rats. Specific bacteria will be implanted into animals to investigate their role in the pathogenesis of these lesions. The possible therapeutic role of certain antibiotics will also be evaluated in animals inflicted with the specific lesions.